This invention relates to a process for treating the dust catch from a cement manufacturing operation and to a system for utilizing the dust from a conventional cement plant operation for producing either cement clinker or a product which is suitable for return to the conventional clinker producing facility. Enriched alkali and sulfur compounds are produced as a biproduct.
Coventional cement clinker producing systems in use today normally consist of a rotary kiln into which raw materials for cement production are supplied to one end and fuel and combustion air supplied to the other end. The system may be in the form of a wet process or dry process and if the dry process may include a preheater system and perhaps a precalcining system. Exhaust gases from the rotary kiln will entrain fine particles of raw material being supplied to the pyroprocessing system of the cement plant. These fine particles of material will be collected in high efficiency dust collectors such as a fabric filter or electrostatic precipitator. The collected particles are referred to herein as "dust catch".
In some instances, this dust catch will be suitable for direct return to the kiln through a dust insufflation system well known in the art. In other applications, the dust catch may be returned to the raw meal blender of the cement plant where it is blended with other incoming feed material and then directed to the clinkering system.
Typically, the oxides of sodium and postassium, commonly referred to as alkalis, and oxides of sulfur contained in the cement raw meal of a conventional cement clinker production facility will be volatilized by the high temperature of the pyroprocessing system and will be concentrated in the dust catch. These compounds must be kept to a minimum in cement clinker and thus must be kept to a minimum in the raw meal. In addition, the dust catch may be comparatively low in certain cement forming compounds. As a result of this concentration of alkali and sulfur compounds and low level of cement forming compounds, the dust catch may be considered to be a waste product. Continued storage of the dust catch in piles adjacent to the cement plant becomes an environmental problem. It would be useful to find a means of utilizing the dust catch to produce a useful product, such as cement clinker, not only to save raw materials, but also to substantially reduce an environmental problem.
If the undesireable alkalis and sulfur compounds can be eliminated from the dust catch, the resultant product can either be used as a component in the manufacture of cement clinker or formed directly into cement clinker.
It should be noted that the dust catch is at least partially calcined material and its use in a cement clinker producing operation will eliminate the need to recalcined a portion of the raw material and thereby reduce the fuel requirement of a cement clinker producing process.
Prior to the present invention, methods were known for utilizing dust catch to produce cement clinker. One such system is shown in my prior U.S. Pat. No. 4,173,487 issued Nov. 6, 1979 wherein the dust catch is chemically adjusted, pelletized and then fed to a rotary kiln. In that process, the kiln is maintained under reducing conditions and at a high back end temperature. While this system produces an acceptable cement clinker from the dust catch of a conventional cement manufacturing operation, its use involves the heavy capital investment of a rotary kiln installation.
Other prior systems for treating cement plant dust catch include the addition of various compounds to the raw material such as the addition of calcium chloride as shown in U.S. Pat. No. 4,404,032 issued Sept. 13, 1983 or the addition of calcium fluoride as shown in U.S. Pat. No. 4,001,030 issued Jan. 4, 1977. Both of these compounds may be less than desireable as a component of the finished cement clinker and may not be readily available materials. The process of the present invention utilizes materials already available in existing cement manufacturing facilities.
Some dust catches are suitable for direct conversion to cement clinker once most of the alkali and sulfur compounds have been volitalized from of the raw mix, although some chemical adjustment may be required. The analysis of other dust catches may be such that it is not suitable to convert the dust catch directly to cement clinker but more appropriate to reduce the alkali and sulfur compounds to produce an intermediate product. This intermediate product is suitable for combination with other raw materials for supply to a conventional cement clinkering operation.
With the present invention, a fluidized bed apparatus is utilized for carrying out the thermal processing of the materials to volitalize alkali and sulfur compounds and produce either an intermediate product which may be used as a component in a conventional cement clinker producing system or the system may be used to produced a final cement clinder product. The product produced will depend upon the analysis of the dust catch being utilized as a feed material and upon the temperature at which the fluid bed will be operated. The alkali and sulfur compounds volatilized are enriched to the point of being a useful byproduct. The use of a fluidized bed system allows an economical system to be provided having lower capital cost then other known systems for treating cement plant dust catch.